There have been used in the past imaging devices using a solid-state image sensing device, such as a digital still camera or the like. Among the imaging devices of this type, the digital still cameras need a highly compact zoom lens system in which a single lens unit is capable of a wide range of imaging function from wide angle to telephoto and has a high performance of imaging that is optimum for high-speed auto-focusing and does not depend upon an object distance.
To meet the above demand, a higher room-ratio zoom lens system has been proposed. Zoom lens systems of this type are disclosed in the Japanese Published Unexamined Patent Application Nos. H04-146407 (Patent Document 1) and H11-174324 (Patent Document 2). Each of the zoom lenses described in these Patent Documents is formed from a group of six lenses, or convex, concave, convex, concave, convex and concave to have an increased zoom ratio.